Commercial and prior art methacrylate adhesive compositions capable of being mixed in a 1:1 ratio and also having practical storage life or shelf life heretofore have not been as elastic or ductile as adhesives with the common but less practical 10:1 mix ratio. This is especially evident upon aging of the adhesive over time or at elevated temperatures. The latter compositions generally employ benzoyl peroxide as the primary initiating species. The earliest and most widely used catalyst combination comprises benzoyl peroxide and a tertiary aromatic amine. The amine is generally incorporated in the polymer and monomer mixture, and the benzoyl peroxide is supplied separately and added to the monomer composition at the time of use. The mixing ratio of monomer composition to peroxide is typically from about 100:1, if pure BPO is used, to about 10:1 or less, if it is diluted with a plasticizer for more convenient mixing. The minimum practical ratio of monomer composition to peroxide component is about 4:1, because the inert plasticizer and other ingredients in the peroxide component have a negative effect on the properties of the cured composition. Benzoyl peroxide is not stable when mixed with methacrylate monomers. Stability of the peroxide catalyst species in admixtures with methacrylate mixtures is necessary for safe and practical storage prior to use in typical 1:1 mix ratio formulations. For this reason, benzoyl peroxide is not a catalyst component in the preferred embodiments of the inventive compositions.
Commercial and prior art methacrylate adhesives with a 1:1 mix ratio typically employ an initiator system comprising a sulfonyl chloride initiating species, a hydroperoxide or other stable non-BPO co-initiating species, and an amine-aldehyde activator. The initiating species are included in the adhesive portion or component A of the adhesive, and the aldehyde-amine activator is included in the activator portion or component B of the adhesive. A common source of sulfonyl chloride is chlorosulfonated polyethylene, sold commercially as Hypalon by duPont. A preferred aldehyde-amine activator is Reillcat ASY-2, a reaction product of butyraldehyde and aniline that is enriched in the content of the active dihydropyridine component, generically referred to as DHP.
While the hydroperoxide/sulfonyl chloride/DHP curing system has provided a means of producing adhesives with a convenient and non-critical 1:1 mix ratio, the resulting formulations have not been preferred for the most demanding structural adhesives applications. This is because these formulations are typically more rigid and may become brittle upon aging relative to compositions cured with BPO and tertiary amines. Therefore, there is a recognized need for adhesives with a convenient 1:1 mix ratio and a high degree of flexibility and retained flexibility after aging. It has now been discovered that specific block copolymers of styrene, butadiene and isoprene can be used to formulate adhesive compositions with these desired characteristics using components that provide a convenient 1:1 mix ratio. Further, in preferred embodiments, these compositions can be formulated without the use of benzoyl peroxide.
U.S. Pat. No. 4,574,142 to Charnock discloses toughened two-part acrylic monomer based adhesive compositions that employ styrene-butadiene (SBS) block copolymer rubbers. Only block copolymers of styrene and butadiene are disclosed, and no preference for the relative ratios of butadiene or styrene, or structure of the polymer is disclosed. An aldehyde-amine component is used as part of the curing catalyst system. However, it is specified that the compositions include a free-radical catalyst system that is free from organic sulfonyl chloride. Sulfonyl chlorides and polymers containing sulfonyl chlorides, such as chlorosulfonated polyethylene, which are preferred accelerators for prior art adhesives, are specifically excluded from the compositions. Polychloroprene, another chlorinated polymer used in prior art compositions, is also disclosed, which has undesirable effects. Thus, Charnock either excludes or teaches away from any chlorine bearing species in the compositions, especially sulfonyl chlorides.
U.S. Pat. No. 5,206,288 discloses improvements in low temperature flexible bond properties with compositions that combine elastomers with low glass transition temperatures Tg and core-shell impact modifiers. In one embodiment employing Kraton in a formulation mixed in an approximately 1:1 mix ratio, the catalyst system comprises BPO paste as an additive in one polymer in monomer component and a tertiary amine, N,N-dimethyl-p-toluidine in the other. BPO is not a preferred component of the compositions of the preferred embodiments.
U.S. Pat. No. 6,989,416 discloses methacrylate adhesives with improved flexibility based on block copolymers of styrene and butadiene. The adhesives contain peroxides and tertiary amine initiators but do not contain sulfonyl chlorides or any other sulfur bearing components or DHP components. Further, they preferably utilize benzoyl peroxide paste as the catalyst component in a mix ratio of 10:1.